Sintered cordierite glass-ceramic bodies

ABSTRACT

The present invention relates to means for sintering powdered glasses having compositions approximating the stoichiometry of cordierite (2MgO.2Al2O3.5SiO2) into strong bodies exhibiting low coefficients of thermal expansion and containing hexagonal cordierite as the crystal phase. Additions of minor amounts of K2O and/or Cs2O to the glass compositions are unique in imparting excellent sinterability at temperatures of 1050*C. and below, and yielding a body which retains a low coefficient of thermal expansion even after repeated thermal cycling to 1000*C.

'United States Patent [191 Miller 5] Dec. 16, 1975 [54] SINTERED CORDIERITE GLASS-CERAMIC 3,480,452 11/1969 Fleischner 106/52 X BODIES 3,490,888 1/1970 Strong 106/52 x [75] Inventor: David M. Miller, Elmira, N.Y. Primary Examiner winston A Douglas [73] Assignee: Corning Glass Works, Corning, Assistant Examiner-John Niebling N Y Attorney, Agent, or Firm-Clinton S. Janes, Jr.;

Cl R. P tt 22 Filed: June 7, 1974 arence a Jr [21] Appl. N0.: 477,197 [57] ABSTRACT The present invention relates to means for sintering 52 US. Cl Powdered glasses having Compositions aPPmXimaFmg E km 2 U Efi ggZ the stoichiometry of cordierite (2MgO.2Al O .5S1O [58] Fwd of 306/39 6 52 into strong bodies exhibiting low coefficients of thermal expansion and containing hexagonal cordierite as [56] References Cited the crystal phase. Additions of minor amounts of K 0 and/or Cs O to the glass compositions are unique in UNITED STATES PATENTS imparting excellent sinterability at temperatures of 2,920,971 1/1960 Stookey lO6/39.6 1050c and below, and yielding a body which retains f; if 1 0 4 a low coefficient of thermal expansion even after re- 1n 0 3,365,314 l/1968 Sack 1 106/52 x peated thermal cychng to 1000 3,450,546 6/1969 Stong 106/52 X 2 Claims, 1 Drawing Figure EXAMPLE I MILLIVOLTS 9 IO ll I2 |3l4 IS IS US Patent Dec. 16, 1975 3,926,648

EXAMPLE 5-- EXAMPLE I SINTERED CORDIERITE GLASS-CERAMIC BODIES 2 for about 16 hours at 1600C. with stirring. Each melt was thereafter poured as a fine stream into a container of water. The glass granules resulting therefrom were dried and then crushed to pass a No. 400 US. Standard Cordierite is a crystalline material demonstrating 5 Sieve (37 microns). Ball milling and fluid energy millrelatively high refractoriness, having amelting point of ing were two techniques utilized in subdividing the about 1460C., and, in a pure form, exhibits excellent granules to a fine powder averaging about l0l5 mielectrical insulating properties and a relatively low crons in diameter. coefficient of thermal expansion over the range up to The glass powders were subsequently pressed into lOO0C., viz., about 16 X 1O /C. However, the sinter- 10 rectangular bars about 3 X /2 X A inch employing water ability of glass powders having the stoichiometry of or a volatile organic liquid as a vehicle and fired to a pure cordierite is very poor. As illustrative of this fact, particular sintering temperature. Since the dimensions the forming of powdered glasses of the cordierite comof the bars were relatively small and the configuration position into bars, which are then fired to temperatures thereof uncomplicated, no special firing technique was up to l200C., results in porous, granular, mechanicalrequired in the sintering step. Simply heating the bars ly-weak ceramic bodies. at furnace rate from room temperature or plunging Therefore, the principal objective of the instant inthem into a furnace operating at a particular temperavention is to provide means for sintering glass powders ture permitted volatilization of the vehicle at a suffihaving compositions approximating the stoichiometry cient rate such that cracking was not observed. of cordierite at relatively low temperatures to yield TABLEI bodies containing hexagonal cordierite as the crystal phase, while maintaining the inherent high refractoril 2 3 4 5 ness of and low coefficient of thermal expansion of sio 50.03% 50.93% 50.9% 51.01% 51.36% pure cordieritg A1203 34.08 34.48 34.6 34.6 34.86

That objective can be achieved utilizing base glasses 58 85; 5' 9 5' approximating the stoichiometry of cordierite (2MgO.- 2 2Al O .5SiO is equivalent, in weight percent, to about Cao 13.6% MgO, 34.9% A1 0 and 51.4% SiO and adding about 0.52.5% K 0 and/or Cs O thereto. Articles formed by the sintering of such glasses at temperatures Table II records the smtermg. procgdure i to below about 1050C. generally about 825-1050C. the examples of Table I along f a Vlsual desimption demonstrate coefficients of thermal expansion over the of fired a charactemauon 9 the slritenng range of 6 of between about l3 18 X behavior exemplified based upon the llnear shrmkage -7/o a use temprature of about 1300C and observed, and a measurement of the thermal expansion o o -7 0 exhibit good thermal stability when repeatedly cycled coeffiplem Over the range 900 10 l between room temperature and looooc exhlblted by the bars as determined by techniques con- Table I reports a group of glasses having p ventlonal for such measurements. X-ray diffraction tions recorded in parts by weight as expressed on the analysls of the .fingl product mdlcated the Presenc? of Oxide basis However inasmuch as the total of the hexagonal cordierite as the sole crystal phase.Ashr1nkrecited components approximates 100, the individual 40 g 8 i 2 j p p h g fi i if amounts can be deemed to essentially reflect percenty o g i g e avmr l n ages. The actual batch ingredients may be any materiec no umace as l 126 pon comp eals, either the oxides or other Compounds which, when tron of the sintering, the electric current to the furnace melted together, will be converted into the desired was cut Off the furnace allowed to .6001 to room oxide in the p p proportion temperature with the bars retained therein.

TABLE II Example No. sintering Treatment Visual Description Sintering Shrinkage Expansion Coefficient 1 Heat at 5C./min. to 980C. White, strong 18% 17 Hold for 1 hour 2 Plunge into furnace at 750C. White, strong 20% 13.3

Hold for 1 hour Heat at 5C./min. to l050C. Hold for 2 hours 3 Plunge into furnace at 750C. White, strong 13% 14.2

Hold for 1 hour Heat at 5C./min. to 1050C. Hold for 2 hours 4 Plunge into furnace at 750C. White, strong 18% 14.5

Hold for 1 hour Heat at 5C./min. to 1050C. Hold for 2 hours 5 Plunge into furnace at 850C. White, weak 2% 11.3

Hold for 1 hour Heat at 5C./min. to l050C. Hold for 2 hours The batch ingredients were blended together in a ball mill to aid in securing a homogeneous melt and then run into platinum crucibles. The crucibles were placed in an electrically-fired furnace and the batches melted Initial sintering of the K 0 and/or cs O-containing glasses can be observed at about 825C. However, long dwell times, e.g., up to 12 hours, may be demanded for substantial sintering to take place. Therefore, temperatures in excess of 900C. are preferred.

The effect of K and Cs O additions on the sinterability of cordierite glasses is readily apparent through a comparison of Examples l-4 with Example 5, a glass consisting essentially of the cordierite stoichiometry. Hence, Examples 1-4 can be sintered to solid, strong bodies at temperatures of 1050C. and below whereas Example is very poorly sintered when fired at 1050C. And, as has been observed above, a glass having the composition of Example 5 will be poorly sintered even after firing at a temperature of 1200C.

A comparison of the coefficient of thermal expansion exhibited in Examples 1-4 with that displayed by Example 5 is also of great interest in that the differences therebetween are relatively slight. This factor is of particular significance since the addition of K O and/or Cs O enables the low temperature sintering of a glass having approximately the cordierite stoichiometry, while retaining the essential physical properties of cordierite.

Table III reports a group of glass compositions based upon the cordierite stoichiometry to which were added small amounts of various oxides to determine the effect thereof upon sinterability and, concomitantly, to learn what effect each addition would have upon the coefficient of thermal expansion. In each instance, the base glass was Example 5 and the examples recorded reflect weight percent additions to that base glass. The batching, melting, and forming parameters employed in producing rectangular glass bars about 3 X /2 X A1. inch were similar to those utilized with Examples l-5 of Table I. In like manner to Table II, a linear shrinkage of at least during the sintering process was deemed 4 also deleteriously affects the electrical properties thereof.

The most optimum properties are secured where the MgO, A1 0 and Si0 contents of the glass closely approximate the stoichiometry of cordierite. Nevertheless, as Tables I and II readily demonstrate, small variations therefrom can be tolerated and very satisfactory properties will be exhibited by the resulting bodies. However, such variations will be maintained within the ranges of about lll6.5% MgO, 31-41% A1 0 and 47-55% SiO with the preferred compositions for refractoriness and low expansion ranging about l2.5-l4.5% MgO, 34-36% A1 0 and 49-52% SiO Likewise, whereas minor additions of extraneous components can be tolerated, the most desirable products will be composed essentially entirely of MgO, A1 0 SiO K 0, and/or Cs O. And, as has been noted above, the inclusion of Li O and/or Na O is particularly discouraged. In sum, the total of all non-essential additions ought not to exceed about 3% by weight.

The mechanism through which K 0 and C5 0 influence the sintering character of the cordierite-stoichiometry glass is not fully understood, but differential thermal analyses (DTA), coupled with X-ray diffraction analyses, of the bodies are believed to provide an explanation. In the appended drawing, the top curve is a DTA of the pure cordierite glass (Example 5) and the bottom curve is a DTA of the Cs O-containing glass, Example 1.

The two exothermic peaks associated with Example 5 are characteristic for most cordierite ceramics produced from glass. The first peak is the result of the formation of the metastable betaquartz solid solution is due to the development of the hexagonal form of desirable to yield a strong body.

TABLE III Example No. Composition Sintering Treatment 6 +0.43% BeO Heat at 5C./minute to 980C. 7 +1.2 MnO Heat at 5C./minute to 980C. 8 +1.2 FeO Heat at 5C./minute to 980C. 9 +1.26% C00 Heat at 5C./minute to 980C. 10 +1.26% NiO Heat at 5C./minute to 980C. 11 +1.34% CuO Heat at 5C./minute to 980C. 12 +1.37% ZnO Heat at 5C./minute to 980C. 13 +0.95% CaO Heat at 5C./minute to 980C. 14 +1.74% SrO Heat at 5C./minute to 980C. 15 +0.59% B 0 Heat at 5C./minute to 980C. 16 +1.53% V 0 Heat at 5C./minute to 980C. 17 +1.28% Cr O Heat at 5C./minute to 980C. 18 +1.35% TiO Heat at 5C./minute to 980C. 19 +1.76% GeO Heat at 5C./minute to 980C. 20 +2.06% ZrO Heat at 5C./minute to 980C. 21 +0.37% Na O Heat at 5C./minute to 980C. 22 Heat at 5C./minute to 980C.

Sintering Shrinkage Strength l3% poor l-3% poor l3% poor l-3% poor l-3% poor l-3% poor l3% poor 1-3% poor 1-3% poor 8% fair 4% poor 8% fair l-2% poor 8% fair 8% fair 8% fair 8% fair +0.25% Li O With the exception of Example 17, the coefficient of thermal expansion over the range of 25-l000C. varied between about 12-20 X 10 /C. The addition of even very small amounts of Cr O appears to cause the expansion to rise over 20 X 10 /C. Unfortunately, the additions giving rise to bodies exhibiting fair strengths, and, hence, fair sinterability, also adversely affected the refractoriness of the final product. And, where even larger additions thereof were made in an attempt to achieve good sinterability, the resulting bodies demonstrated use temperatures below 1150C. and/or coefficients of thermal expansion far in excess of 20 X l0' /C. Finally, the inclusion of Li O and/or Na O not only reduces the refractoriness of the final product but cordierite. From the bottom curve, it appears that the addition of Cs O suppresses the formation of the betaquartz solid solution phase and crystallization occurs only at the higher temperatures. Hence, it is believed that the developments of beta-quartz solid solution and hexagonal cordierite are superimposed at the higher temperature exotherm. This phenomenon has the further effect of stabilizing the glass against crystallization in the temperature range where glass sintering is initiated. Hence, sintering in Example 1 has been demonstrated to commence at about 830C. When held at that temperature, the glass will sinter for about 9-13 hours before substantial devitrification and stiffening of the body will occur.

I claim:

6 A1 0 47-55% SiO and 0.5-2.5% K 0 and/or Cs O. 2. A sintered glass-ceramic body according to claim 1 consisting essentially, by weight on the oxide basis, of about 12.5-l4.5% MgO, 34-36% A1 0 49-52% SiO and 0.5-2.5% K 0 and/0r Cs O. 

1. A SINTERED GLASS-CEMAMIC BODY CONTAINING HEXAGONAL CORDIERITE AS THE CRYSTALLINE PHASE, EXHIBITING A COEFFICIENT OF THERMAL EXPANSION (25*-1000*C.) BETWEEN ABOUT 13-18 10-7/*C., AND DEMONSTRATING A USE TEMPERATURE UP TO 13000*C. CONSISTING ESSENTIALLY, BY WEIGHT ON THE OXIDE BASIS, OF ABOUT
 2. A sintered glass-ceramic body according to claim 1 consisting essentially, by weight on the oxide basis, of about 12.5-14.5% MgO, 34-36% Al2O3, 49-52% SiO2, and 0.5-2.5% K2O and/or Cs2O. 